Guessing the timezone identifier from just an abbreviation + UTCoffset + DSTenabled, is a tricky thing to do at first, as a combination is not unique.
It is even trickier to do with *just* UTCoffset + DSTenabled.

The fallback map was only really implemented to keep function parity with old PHP versions. At the time when it was written, there was no timezone that used UTC+6 (without DST), and hence, it does not have an entry. Right now for example, there could be several equally correct pickings:

derick@whisky:~ $ for i in /usr/share/zoneinfo/Asia/*; do zdump -v $i -c 2013,2018 |grep "isdst=0 gmtoff=21600"; done
/usr/share/zoneinfo/Asia/Barnaul  Sat Oct 25 19:00:00 2014 UT = Sun Oct 26 01:00:00 2014 +06 isdst=0 gmtoff=21600
/usr/share/zoneinfo/Asia/Barnaul  Sat Mar 26 19:59:59 2016 UT = Sun Mar 27 01:59:59 2016 +06 isdst=0 gmtoff=21600
/usr/share/zoneinfo/Asia/Novosibirsk  Sat Oct 25 19:00:00 2014 UT = Sun Oct 26 01:00:00 2014 NOVT isdst=0 gmtoff=21600
/usr/share/zoneinfo/Asia/Omsk  Sat Oct 25 19:00:00 2014 UT = Sun Oct 26 01:00:00 2014 OMST isdst=0 gmtoff=21600
/usr/share/zoneinfo/Asia/Tomsk  Sat Oct 25 19:00:00 2014 UT = Sun Oct 26 01:00:00 2014 +06 isdst=0 gmtoff=21600
/usr/share/zoneinfo/Asia/Tomsk  Sat May 28 19:59:59 2016 UT = Sun May 29 01:59:59 2016 +06 isdst=0 gmtoff=21600
/usr/share/zoneinfo/Asia/Yekaterinburg  Sat Oct 25 19:59:59 2014 UT = Sun Oct 26 01:59:59 2014 YEKT isdst=0 gmtoff=21600

I am reluctant to add more entries to this fallback map, and even if we would add it, I think it should be added as:

    { "novt",  0,  360, "Asia/Novosibirsk" },

To fit in with:

    { "novst", 1,  420, "Asia/Novosibirsk" },
    { "krat",  0,  420, "Asia/Krasnoyarsk" },
    { "krast", 1,  480, "Asia/Krasnoyarsk" },

Because of the non-uniqueness, I am not in favour in adding more entries.

As for the naming of zone is up to you. For me important only valid result from function.

What language capabilities I can to use for maping offset to zonename?

Of course I can build my own map, but it's better to use PHP capabilities for me.

Related: isdst=-1 is not supported for the fallback search. https://3v4l.org/coVcm

@derick: Can the search be rewritten to work with just the main timelib_timezone_lookup map? I'm too lazy to make a patch or PR for this (let alone test it) but I'm thinking like
https://gist.github.com/requinix/dffeb6a11b278182901cd277bc5b6ba8
And I don't see anything else using timelib_timezone_fallbackmap/fallbackmap.h so they could go away.

Changed expected timezone to "Asia/Novosibirsk" in PR.

@requinix@php.net — yes, we can remove the use of the fallback map, but not in any released branch as it would be a BC break. Also note that changes to timelib need to go to https://github.com/derickr/timelib — PHP isn't the only user of this library.